What's the date, in Usenet time?

Python 3, 109 bytes

from datetime import*
i=input()
z=date(*map(int,i.split())).toordinal()-727806
print([i,'1993 09 %d'%z][z>9])

Try it online!

-59 bytes thanks to notjagan
-3 bytes thanks to Mr. Xcoder
-2 bytes thanks to officialaimm
-12 bytes thanks to Jonathan Allan

If[(s=#&@@{1993,9}~DateDifference~#)>0,{1993,9,s+1},#]&

I/O

{2017, 8, 6} ->{1993, 9, 8741}
{1986, 1, 28}->{1986, 1, 28}

-6 bytes thanx to user202729

f=
s=>(d=new Date(s)/864e5-8643|0)>9?'1993-09-'+d:s

<input size=10 oninput=o.textContent=/\d{4}(-\d\d){2}/.test(this.value)?f(this.value):``><pre id=o>

Based on @Mr.Xcoder's algorithm.